AD8336ACPZ-R7 datasheet(22/28 Pages) AD

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AD8336

Rev. 0 | Page 22 of 28

SETTING THE GAIN

NOISE

The noise of the AD8336 is dependent on the value of the VGA

gain. At maximum VGAIN, the dominant noise source is the

preamp but shifts to the VGA as VGAIN diminishes.

The overall gain of the AD8336 is the sum (in dB) or the

product (magnitude) of the preamp gain and the VGA gain.

The preamp gain is calculated as with any op amp, as seen in

the Applications section. It is most convenient to think of the

device gain in exponential terms (that is, in dB) since the VGA

responds linearly-in-dB with changes in control voltage VGAIN at

the gain pins.

The input referred noise at the highest VGA gain and a preamp

gain of 4×, with RFB1 =100 Ω and RFB2 = 301 Ω, is 3 nV/

√Hz, and

determined by the preamp and its gain setting resistors. See

Table 4 for the noise components for the preamp.

The gain equation for the VGA is

Table 4. AD8336 Noise Components for Preamp Gain = 4×

(V)

(dB)

⎥

⎦

⎤

⎢

⎣

⎡

AIN

Gain

VGA

Noise Component

Noise Voltage (nV/√Hz)

Op Amp (Gain = 4×)

2.6

RFB1 = 100 Ω

0.96

RFB2 = 301 Ω

0.55

VGA

0.77

where VG = VGPOS − VGNEG

The gain and gain range of the VGA are both fixed at 34 dB and

60 dB, respectively; thus, the composite device gain is changed

by adjusting the preamp gain. For a preamp gain of 12 dB (4×),

the composite gain is −14 dB to +46 dB. Thus, the calculation

for the composite gain (in dB) is

Using the listed values, the total noise of the AD8336 is slightly

less than 3 nV/

√Hz, referred to the input. Although the output

noise VGA is 3.1 nV/

√Hz, the input referred noise is 0.77 nV/√Hz

when divided by the preamplifier gain of 4×

/V]

(V)

[

PRA

Gain

Composite

At other than maximum gain, the noise of the VGA is determined

from the output noise. The noise in the center of the gain range

is about 150 nV/

√Hz. Since the gain of the fixed gain amplifier

that is part of the VGA is 50×, the VGA input referred noise is

approximately 3 nV/

√Hz, the same value as the preamp and

VGA combined. This is expected since the input referred noise

is the same at the input of the attenuator at maximum gain.

However, the noise referred to the VGAI pin (the preamp

output) increases by the amount of attenuation through the

ladder network. The noise at any point along the ladder

network is primarily comprised of the ladder resistance noise,

the noise of the input devices, and the feedback resistor network

noise. The ladder network and the input devices are the largest

noise sources.

For example, the midpoint gain when the preamp gain is 12 dB is

/V]

[

Figure 3 is a plot of gain in dB vs. VGAIN in mV, when the

preamp gain is 12 dB (4×). Note that the computed result

closely matches the plot of actual gain.

In Figure 3, the gain slope flattens at the limits of the VG input.

The gain response is linear-in-dB over the center 80% of the

control range of the device. Figure 78 shows the ideal gain

characteristics for the VGA stage and composite VGA + preamp.

VG (V)

–10

FOR PREAMP GAIN = 26dB

–30

–20

FOR PREAMP GAIN = 6dB

GAIN CHARACTERISTICS

COMPOSITE GAIN

VGA STAGE GAIN

USABLE GAIN RANGE OF

8336

FOR PREAMP GAIN = 12dB

0.5

0.7

0.3

0.1

–0.1

–0.3

–0.5

–0.7

At minimum gain, the output noise increases slightly to about

180 nV/

√Hz because of the finite structure of the X-AMP.

OFFSET VOLTAGE

Extensive cancellation circuitry included in the variable gain

amplifier section minimizes locally generated offset voltages.

However when operated at very large values of gain, dc voltage

errors at the output can still result from small dc input voltages.

When configured for the nominal gain range of −14 dB to 46 dB,

the maximum gain is 200× and an offset of only 100 μV at the

input generates 20 mV at the output.

The primary source for dc offset errors is the preamplifier;

ac coupling between the PRAO and VGAI pins is the simplest

solution. In applications where dc coupling is essential, a

compensating current can be injected at the INPN input (Pin 5)

to cancel preamp offset. The direction of the compensating

current depends on the polarity of the offset voltage.

Figure 78. Ideal Gain Characteristics of the AD8336

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